The present invention generally relates to photoelectric conversion devices and more particularly to a photosensor having an amorphous silicon photoabsorption layer for photoelectric conversion.
Photosensors are used in various technical fields such as image sensors. The photosensors using amorphous silicon provides a particularly preferable feature of improved sensitivity due to the high efficiency of photoelectric conversion that is pertinent to the amorphous silicon band structure. However, the photosensor of this type has a problem of forming afterimages or decay lag, and there is a demand to suppress such a decay lag as much as possible. Further, to achieve high resolution images including a large number of picture elements, photosensors have to be assembled on a common substrate as an integrated circuit with high integration density. In such an integrated circuit of photosensors, it is necessary to eliminate the leak current between adjacent pixels.
FIG. 1 shows a typical conventional photosensor employing amorphous silicon for the photoabsorption layer.
Referring to FIG. 1, the photosensor comprises a silicon substrate 1 covered by a silicon oxide insulator layer 2. On the insulator layer 2, there is provided an insulator layer 4 of phosphosilicate glass (PSG) except for an opening 4a that exposes a part of the insulator layer 2. On the exposed part of the insulator layer 2, there is provided a lower electrode structure 3 that includes a layer of aluminum alloy and a titanium nitride (TiN) layer 3B serving for the diffusion barrier.
Further, an undoped, intrinsic type silicon carbide layer 5 of amorphous phase is provided on the electrode structure 3. On the electrode structure 3, there is provided an amorphous silicon photoabsorption layer 6 as an essential element for the photoelectric conversion. Further, an amorphous silicon carbide layer 7 doped to the p.sup.+ -type is provided on the amorphous silicon layer 6, and an upper, transparent electrode layer 8 of so-called ITO (indium tin oxide) is provided further on the silicon carbide layer as an electrode opposing the pixel electrode 3A. Thereby, a photodiode is formed between the electrode 3 and the electrode 8. In the illustrated example, the amorphous silicon layer 6 is deformed by a side wall 6a together with the silicon carbide layers 5 and 7 such that the amorphous silicon layer 6 is separated from other amorphous silicon layer forming adjacent photodiodes. In response to the patterning of the layers 5-7, the ITO layer 8 is also patterned.
FIG. 2 shows the band structure of the photosensor of FIG. 1.
In use of the device of FIG. 1, an acceleration voltage is applied across the upper electrode 8 and the lower electrode 3 and thereby there is formed a gradient of potential or electric field in the amorphous silicon layer 6 as shown by the sloped conduction band E.sub.C and the valence band E.sub.V. Upon incidence of an optical beam, there are formed electron-hole pairs in the silicon layer 16 due to the inelastic scattering of photons, and the electrons thus formed are collected by the lower electrode 3 after moving through the silicon layer 6 and crossing through the silicon carbide layer 5 along the sloped conduction band. On the other hand, the holes are collected by the ITO electrode after passing through the amorphous silicon layer 6 and the silicon carbide layer 7 along the sloped valence band. Thereby, the photodiode causes a photoelectric current to flow, and the intensity of the current thus obtained is generally proportional to the intensity of the light beam, i.e., the number of photons in the light beam. Thus, the detection of incidence of the light beam is achieved.
In the structure of FIG. 1, the injection of unwanted electrons into the amorphous silicon layer from the ITO electrode 8 is effectively prevented by the potential barrier formed in the amorphous silicon carbide layer 7. Thus, the silicon carbide layer 7 contributes to decrease the dark current of the photodiode. On the other hand, the lower amorphous silicon carbide 5 contributes to prevent the reaction between the amorphous silicon in the layer 6 and the aluminum electrode 3 at the time of fabrication of the device.
In this conventional photosensor, there is a problem in that, because of the high resistivity of the undoped silicon carbide layer 5, a large electric field develops in the silicon carbide layer 5 as can be seen in the steep slope of the band diagram of FIG. 2. Such a large electric field in the silicon carbide layer 5 inevitably reduces the electric field in the amorphous silicon layer 6 and thereby the acceleration of the electrons and holes in the amorphous silicon layer tends to be made insufficiently. Such an insufficient electric field increases the decay lag when the photodiode is used as an image sensor for reading images. It is known that an electric field of at least 4 volts/.mu.m is needed in order to satisfactorily suppress the decay lag (Kuriyama H., et al. "Suppression of the Decay Lag of a-Si Photodiodes," Abstract of 1989th annual meeting, The Institute of Television Engineers of Japan, pp. 7-8, July 19-21, 1989, Osaka).
The foregoing problem of poor decay lag may be eliminated when doped silicon carbide is used for the silicon carbide layer 5 such that the silicon carbide layer 5 becomes conductive. However, such a doping of the silicon carbide, rendering the silicon carbide layer 5 conductive, inevitably invites leakage of electric current between adjacent pixels. In order to prevent such leakage current to flow, it is necessary to divide the photosensor into a number of isolated pixels by patterning the amorphous silicon layer 6 as shown in FIG. 1 together with the silicon carbide layers 5 and 7 and the ITO electrode 8 thereon. However, such a process of patterning, including the etching process of silicon, silicon carbide and oxide, increases the number of steps necessary for fabricating the photosensor and hence its cost.